OSI Layers

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it. On each layer, an instance provides services to the instances at the layer above and requests service from the layer below. OSI reference model is now considered as a primary standard for internetworking and inter computing. Today many network communication protocols are based on the standards of OSI model.

The 7 layers of the OSI models can be divided into upper and lower layers.

Layer 7: Application Layer
The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This is the layer that actually interacts with the operating system or application whenever the user perform network-related activities.  

Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.

Network Devices: Gateway network device is operated on the application layer.

Layer 6:Presentation Layer
Layer 6 takes the data provided by the Application layer and converts it into a standard format that the other layers can understand.

Network Devices: Gateway Redirector is operates on the presentation layer.

Layer 5: Session Layer
It establishes, manages and terminates the connections between the local and remote application


Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, RPC
Network Devices: Gateway

Layer 4: Transport Layer

Protocols: These protocols work on the transport layer TCP, SPX, NETBIOS, ATP and NWLINK.
Network Devices: The Brouter, Gateway and Cable tester work on the transport layer.

Layer 3: Network Layer

Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.
Network Devices: Network devices including Router, Brouter, Frame Relay device and ATM switch devices work on the network layer.

Layer 2:Data Link Layer

Protocols: Logical Link Control ,  Media Access Control
Network Devices: Bridge , Switch, ISDN Router, Intelligent Hub, NIC, Advanced Cable Tester

Layer 1: Physical Layer

Protocols: Protocols that work on the physical layer are ISDN, IEEE 802 and IEEE 802.2
Network Devices: Hubs, Repeaters, Oscilloscope and Amplifier works on the network devices.